Late Cretaceous, island-arc type, magmatic evolution resulted in the formation of the Srednogorie volcanic intrusive zone. The Chelopech mineral deposit is located within the Panagyurishte metallogenic district, a central part of the Srednogorie zone.

The Chelopech region consists of a Precambrian metamorphic basement consisting of gneisses, amphibolites, and metasediments overlain by Upper Cretaceous, volcanosedimentary sequences which include the Chelopech formation; the primary host to mineralisation.

The Chelopech Formation reaches thicknesses of up to 2,000 m and consists of Lower and Upper units.

The Lower Chelopech Unit is comprised of a basal sequence of siltstones and calcareous argillites with subordinate terrigenous sandstones and angular conglomerates. Upwards these sediments become intercalated and eventually superseded by volcanic sequences including andesites, andesitic agglomerates, andesitic lapilli and psammitic tuffs.

The Upper Chelopech Unit of Coniacian-Santonian age (Lower Senonian) comprises a complex of andesitic and dacitic lavas and tuffs with siliciclastic, volcaniclastic and argillaceous sediments intruded by sub-volcanic bodies of porphyritic andesites. The Upper Chelopech Formation passes up and laterally from mixed (terrigenous-volcanogenous) gritty sandstones, with volcanogenic exhalative iron-manganese oxide horizons, into volcanogenic talus breccias and agglomeratic tuffs of andesitic affinity.

Three successive mineralisation stages have been recognised at Chelopech, including an early Fe-S stage consisting mainly of disseminated and massive pyrite, a second Cu As-S stage which is the economic Cu and Au stage, and a late Pb-Zn stage. These display different geometries, including veins, breccias, massive and disseminated sulphides.

The mineralisation occurs in a range of different morphologies, including lens-like, pipe-like and columnar bodies that typically dip steeply towards the south. The mineralised zones vary from 40 to 200 m in length, are 20 to 130m thick, and can extend at least 390 m down plunge.

Production from underground is attained via Sublevel Longhole Open Stoping. The various ore bodies are developed at nominal 30 m vertical intervals and accessed by major declines in both, the Western and Central areas. Stopes are designed to be 20 m wide between the levels. The length of the stope depends on the geotechnical conditions, but can range between 20 and 60 m. The new trend of stope design is to keep a 20-30m length and 60m height, where geological and geotechnical conditions are suitable. This allows for improvement in ore handling and dust suppression during ore mucking as a result of shorter remote loading.

Once mined via an “end-slice” methodology, stopes are backfilled with “paste fill” produced from the mill tailings, to which cement is added and which is gravity fed underground via a system of borehole and pipes to the stopes being filled.

Underground Crusher Conveyor System
The previous system of train haulage and shaft hoisting has been entirely replaced by a 2 Mt/a underground crushing and conveying system, that takes ore from an ore pass system underground reporting to the 195 level and crushes, transports by conveyors and discharges the ore onto a 6,000 t live capacity reclaim stockpile on surface.

This ore handling system incorporates a primary crusher (a 1,070 mm x 1,500 mm jaw crusher) between the 195 level and the 165 level underground, which discharges into a 400 t crushed ore bin. The crusher is fed from a ROM bin sitting under a grizzly with openings of 800 mm x 800 mm.

A plate feeder draws material from the 400 t crushed ore bin and loads a picking belt (CV1) for removal of tramp metal using a self-cleaning magnet. Material is then conveyed via six more haulage conveyors (CV2- CV7) to the surface. The surface conveyor (C1105) transfers this material to the surface reclaim stockpile, where it is reclaimed and conveyed to the SAG mill to provide uninterrupted feed to the process plant.

The total length of the six underground conveyors is 3.9 km. Total lateral development required was 4.5 km. The design capacity of the system is 400 t/hr as a result of this underground crusher installation, the existing surface crusher installation is reduced to one crusher, to handle oversize and to maintain minimum production in case of emergency.

The new conveyor system was commissioned in 2012 and has been in operation continuously since then. This system is the only means of ore hoisting to surface.

Current ore treatment processes comprise conventional crushing of run-of-mine (ROM) ore in a primary jaw crushing circuit, grinding in a SAG milling circuit, bulk floatation, three-stage cleaner flotation and concentrate dewatering to produce the copper/gold concentrate, while the pyrite is recovered from the copper circuit cleaner tails.

The primary saleable product is a gold-copper concentrate containing, on average, 16.5% Cu, 35 g/t Au and 5.5% arsenic, which is loaded at the mine site through a conveyor system from the stockpile into rail wagons and dispatched to the Port of Burgas for sea transportation to the Company’s smelter in Namibia and to third parties.

Since 2014, pyrite concentrate, containing gold, has been produced in a section with a capacity allowing the production of up to 400,000 tonnes of pyrite concentrate per year from the mill feed as a separate secondary concentrate product, in addition to the produced gold-copper concentrate. Production i ........